1. Field of the Invention
The present invention relates to a non-volatile memory device with a large capacity and a high density. More particularly, the present invention relates to a non-volatile memory device adapted for use in a computer, a memory card, a wordprocessor, and the like.
2. Description of the Prior Art
As non-volatile memory devices, the following four are well known and widely used:
(1) Magnetic tape, PA1 (2) Magnetic disc, PA1 (3) IC non-volatile memory such as EPROM and EEPROM, and PA1 (4) Optical magnetic disc.
Each of these devices will be more particularly described.
(1) Magnetic tape PA0 (2) Magnetic disc PA0 (3) EPROM and EEPROM PA0 (4) Optical magnetic disc PA0 (1) Large capacity and high density PA0 (2) Resistance against impact and vibration PA0 (3) Compact, simple and inexpensive device for writing and reading PA0 (4) High-speed writing and reading
A magnetic tape is a most typical rewritable non-volatile memory device. It is in wide use for an audio tape and a video tape for the low price thereof, and also is used for a back-up memory of a computer for a very large capacity thereof.
The magnetic tape has disadvantages of performing only sequential data writing and reading and of long access time.
A magnetic disc is generally used for an external memory device of a computer or a wordprocessor. Magnetic discs include a floppy disc which is easy to handle and inexpensive and a hard disc which has a larger capacity but is harder to handle and more expensive than a floppy disc.
A magnetic disc has advantages of a high-speed random access and relatively easy data writing and rewriting.
The magnetic disc has a limit in improving the capacity and the density. For example, the capacity of a 3.5-inch floppy disc is approximately 1 megabyte, and that of a 3.5-inch hard disc is approximately 40 megabytes.
EPROM and EEPROM are representative IC non-volatile memory devices, which are rewritable and are capable of high density writing. In the EPROM, writing is electrically effected and erasing is effected by ultraviolet irradiation. In the EEPROM, both writing and erasing are electrically effected. These IC non-volatile memory devices have advantages such as compactness, lightness, short access time and low power consumption.
The EEPROM, which is capable of electrical writing and erasing, will be described in detail. FIG. 28 is a cross sectional view of a memory cell of an EEPROM. The memory cell includes a silicon substrate 7, a gate oxide film 5 provided on the silicon substrate 7, and a floating gate 4 and a control gate 2 provided on the gate oxide film 5. The floating gate 4 has a function of accumulating and keeping carriers. The control gate 2 has a function of controlling injection of the carriers to the floating gate 4. The floating gate 4 and the control gate 2 are insulated from each other by an insulating film 3 formed of silicon oxide. On the silicon substrate 7, a surface passivation film 1 is provided so as to cover the floating gate 4 and the control gate 2. The surface passivation film 1 is usually formed of silicon oxide or silicon nitride. The silicon substrate 7 has a source area 8 and a drain area 6 formed by impurity injection at upper portions thereof. A channel area 9 is provided between the source area 8 and the drain area 6.
For writing data in an EEPROM having the above construction, a voltage is applied between the drain area 6 and the control gate 2, and carriers are injected to the floating gate 4 through the gate oxide film 5. For erasing the data, a voltage is applied between the source area 8 and the control gate 2, and the carriers are removed utilizing the Fowler-Nordheim (N-F) Tunneling phenomenon. For reading the data, ON or OFF is judged based on a threshold of the inverting voltage at the channel area 9 between the source area 8 and the drain area 6.
In the above construction, since carrier injection and removal are done through the gate oxide film 5, the quality and the thickness of the gate oxide film 5 are very important. In an EEPROM having a memory capacity of 1 megabit, the gate oxide film 5 usually has a thickness of approximately 20 nm. Accordingly, it is difficult to control the quality and the thickness thereof, causing a serious problem that the production cost is increased due to the decline of the yield. Both long and short sides of an EEPROM chip are usually 7 to 10 mm. An enlargement of the size in order to increase the memory capacity lowers the yield and thus raises the production cost.
For the above problems, the EEPROM is limited in improving the memory capacity. The average memory capacity of the EEPROMs used today is approximately 1 to 4 megabits, which is smaller than the other non-volatile memory devices such as a magnetic disc and an optical magnetic disc.
An optical magnetic disc capable of rewriting is one of the representative large capacity non-volatile memory devices.
FIG. 29 shows a construction of an optical magnetic disc. The optical magnetic disc includes magnetic thin films 15 and 16 as memory mediums. The magnetic thin films 15 and 16 show perpendicular magnetization. For writing data, a laser beam 20 is converged to a convergence area 21 in a weak magnetic field which has an opposite polarity to that of the magnetic field to which the magnetic thin films 15 and 16 have been magnetized. Data is written in the magnetic thin films 15 and 16 by local heating. Data reading is effected utilizing the Kerr effect or the Faraday effect. In more detail, when the laser beam 20 which is linearly polarized is emitted to the disc, the plane of polarization of light transmitted through or reflected by the disc is rotated in accordance with the condition of magnetization of the magnetic thin films 15 and 16. Such a rotation of the plane of polarization is converted into an optical signal using an analyzer and then is detected by a photo detector as an electric signal. .Thus, the data is read out. The optical magnetic disc is practically used for a large capacity memory device for document files and image files.
In the optical magnetic disc, writing can be done without contact by emitting the laser beam 20 through a transparent glass substrate 12. Accordingly, dust on a writing plane 23 causes no problem. Since the laser beam 20 is not focused on a surface 22 of the glass substrate 12, the laser beam 20 has a large diameter of several hundred microns at the surface 22. Accordingly, the presence of dust here does not have any serious affect.
Owing to the writing and reading by use of the converged laser beam 20, high density writing is realized. For example, a 3.5-inch disc has a large capacity of approximately 120 megabytes.
A disadvantage of the optical magnetic disc is that a rotating mechanism required for rotating the disc enlarges the writing and reading apparatus and thus increases the production cost.
Conventional rewritable non-volatile memory devices have the above-mentioned advantages and disadvantages. An ideal non-volatile memory device must fulfill the following four requirements, which cannot be achieved by any conventional device for the following reasons.
A floppy disc cannot meet this requirement as is apparent from the fact that a 3.5-inch floppy disc has a capacity of only 1 megabyte.
IC non-volatile memory devices such as an EPROM or an EEPROM can realize high density, but not a large capacity due to the restriction of the area thereof.
In the case of a hard disc, a large capacity can be realized by integrating a plurality of discs. In this case, however, the distance between a head and the disc is as microscopic as 1 micron or less. Such a device is easily destroyed by impact, vibration, or even microscopic dust adhering on the head or the disc.
Since a floppy disc, a hard disc and an optical magnetic disc effect writing and reading by rotating a disc, a rotating mechanism such as a motor is required. Accordingly, the writing and reading apparatus is inevitably large and complicated.
In the case of a hard disc, an alleviating material is required in order to guarantee a precise distance between the disc and the head and resistance against impact. The alleviating material enlarges and complicates the writing and reading apparatus.
An optical magnetic disc also requires a large and complicated writing and reading apparatus because of the use of a laser and a magnet.
A floppy disc, a hard disc and an optical magnetic disc have a limit in enhancing a reading speed thereof since data is searched by rotating the disc. A magnetic tape is especially slow in writing and reading.
Since no conventional non-volatile memory device fulfills all the four requirements, a completely novel device has been demanded.